Titanium alloys are light in weight and yet high in strength and excellent in corrosion resistance, so are being applied in various fields. Among these, α+β-type titanium alloys such as Ti-6Al-4V are superior in the balance of strength, ductility, toughness, and other mechanical properties, have been widely used in the past in the aerospace field, and in recent years have increasingly been applied to auto parts.
However, with an Ti-6Al-4V-based alloy, V is expensive, so alloys to which Fe is added as an alternative element to V have been studied for a long time now. For example, the Ti-5Al-2.5Fe-based alloy described in “Titanium Science and Technology” (issued 1984 by Deutsche Gesellschaftfur Metallkunde E.V.), p. 1335, the Ti-6Al-1.7Fe-0.1Si-based alloy described in “Advanced Materials & Process” (issued in 1993), p. 43, etc. are being studied.
Japanese Patent Publication (A) No. 07-062474 discloses as an alloy superior in hot-rollability and cold-rollability an α+β-type titanium alloy comprising, by mass %, Fe: 1.4% to less than 2.1%, Al: 4% to less than 5.5%, and a balance of titanium and unavoidable impurities.
Japanese Patent Publication (A) No. 03-197635 proposes as a titanium alloy superior in heat resistance an α+β-type titanium alloy containing, by mass %, Al: 2 to 7%, V: 2 to 12%, and Mo: 1 to 7%, further containing one or more of Sn: 1 to 6%, Zr: 3 to 8%, Fe: 0.1 to 3%, and Cu: 0.1 to 3%, comprising a balance of Ti and unavoidable impurities, and having one or more of P, As, Sb, Bi, S, Se, and Te added in a total of 10 to 104 ppm.
Japanese Patent Publication (A) No. 2003-201530 proposes a high strength titanium alloy superior in hot-rollability containing, by mass %, Al: 3 to 7%, C: 0.08 to 0.25%, and at least one of Mo, V, Cr, Fe in an Mo equivalent of 3 to 10%.
Japanese Patent No. 2606023 proposes a method of production of a high strength, high toughness a+13 titanium alloy containing Al: 3 to 7%, V: 2.1 to 5.0%, Mo: 0.85 to 3.15%, Fe: 0.85 to 3.15%, and O: 0.06 to 0.20%.
Japanese Patent Publication (A) No. 2000-273598 proposes a method of production of a high strength coil cold-rolled titanium alloy containing an Al equivalent of 3 to 6.5%, at least one type of complete solid solution β-stabilizing element in an Mo equivalent of 2.0 to 4.5%, and a eutectoid β-stabilizing element in an Fe equivalent of 0.3 to 2%.
Further, Japanese Patent Publication (A) No. 2000-204425 proposes a high strength, high ductility α+β-type titanium alloy containing at least one type of complete solid solution β-stabilizing element in an Mo equivalent of 2.0 to 4.5% and at least one type of eutectoid β-stabilizing element in an Fe equivalent of 0.3 to 2.0% and an Al equivalent of 3 to 6.5% and, further, Si in an amount of 0.1 to 1.5%.
However, the Ti-5Al-2.5Fe-based alloy described in “Titanium Science and Technology” (issued 1984 by Deutsche Gesellschaft fur Metallkunde E.V.), p. 1335 and the Ti-6Al-1.7Fe-0.1Si-based alloy described in “Advanced Materials & Process” (issued 1993), p. 43 are somewhat smaller in hot deformation resistance than an Ti-6Al-4V-based alloy and just somewhat superior in hot-rollability. Further, they have the problem that the strength is also insufficient.
Further, the alloy described in Japanese Patent Publication (A) No. 07-062474 has a tensile strength of less than 1000 MPa. It cannot be said to have a sufficient strength. There is the problem that the hot-rollability and room temperature ductility and the cold-rollability are insufficient.
On the other hand, the alloy described in Japanese Patent Publication (A) No. 03-197635 has fine amounts of P, As, Sb, Bi, S, Se, Te, and other elements with larger valence electron number than Ti added to it so as to suppress the growth of the high temperature oxide layer, but there is the problem that these additive elements do not have any particular effect on the strength or on the hot-rollability and room temperature ductility and the cold-rollability.
The alloy described in Japanese Patent Publication (A) No. 2003-201530 contains the α-stabilizing element C as an element increasing the strength from room temperature to the 500° C. level in temperature range and not having an effect on the hot-rollability. This C lowers the hot deformation resistance, but inhibits the room temperature ductility and cold-rollability.
The alloy described in Japanese Patent No. 2606023 includes expensive V in an amount of 2.1 to 5.0%, so is insufficient as a low cost α+β alloy for replacing Ti-6Al-4V. Further, it is desirable that the hot-rollability as well be equivalent to that of Ti-6Al-4V and further that a superior workability be imparted.
Japanese Patent Publication (A) No. 2000-273598 describes a method of production of a coil cold-rolled titanium alloy containing an Al equivalent in an amount of 3 to 6.5%, at least one type of complete solid solution β-stabilizing element in an Mo equivalent of 2.0 to 4.5%, and a eutectoid β-stabilizing element in an Fe equivalent of 0.3 to 2%. Specifically, it describes a specific alloy composition constituted by Ti-(4 to 5%)Al-(1.5 to 3%)Mo-(1 to 2%)V-(0.3 to 2.0%)Fe. The alloy of the above alloy composition has to include V, so there are the problems that the alloy is insufficient compared with Ti-6Al-4V in terms of the cost and in terms of the hot-rollability.
The alloy described in Japanese Patent Publication (A) No. 2000-204425 is a titanium alloy containing an Al equivalent of 3 to 6.5%, at least one type of complete solid solution β-stabilizing element in an Mo equivalent of 2.0 to 4.5%, and a eutectoid β-stabilizing element in an Fe equivalent of 0.3 to 2.0% and further containing Si in 0.1 to 1.5%, but if including Si in an amount of 0.1% or more, Ti and Si compounds precipitate at the interface between the α-phase and the β-phase causing the problem of deterioration of the fatigue characteristics or the room temperature ductility and cold working characteristics.
Further, in applications of use at undersea oil fields and other high temperature, high pressure, highly corrosive extreme environments, there is the problem that all of the above alloys are insufficient in corrosion resistance in some cases.